1. Field of the Invention
This invention relates to a no-charge-exchange transfer stations in a xerographic copying machine. More particularly, this invention relates to neutralizing toner particles as a part of the no-charge-exchange transfer process.
2. Review of Prior Art
Transfer stations in the xerographic process typically employ a transfer corona, a charge-exchange transfer roller or a no-charge-exchange transfer roller. Transfer coronas are placed on the side of the paper opposite from the toner. The corona charges the paper and charge on the paper causes the toner to transfer from a photoconductor to the paper. Neutralizing coronas to partially neutralize the paper are known in transfer corona stations. With transfer coronas the problem is that by charging the paper to accomplish the transfer the paper is also tacked to the photoconductor. To aid separation of the paper or sheet from the photoconductor a neutralizing corona is used to partially discharge the paper after the transfer of toner and just prior to separation of the paper from the photoconductor. Charge-exchange transfer rollers operate in substantially the same manner as transfer coronas.
The problem solved by this invention occurs in no-charge-exchange (NCX) transfer rollers. In NCX transfer rollers the toner is transferred without charging the paper or sheet to which the toner is transferred. One description of this process appears in commonly assigned U.S. Pat. No. 3,879,121. The problem arises in that since the copy paper or transfer sheet is uncharged the only forces holding the toner on the sheet after the sheet leaves the transfer roller are the forces of adhesion. While the sheet is on the transfer roller the electric field from the roller assists in holding the toner particles on the roller. After the sheet leaves the roller the only substantial charges present on the sheet are the charges on the toner particles. Thus, the electrical forces on the toner particles causing the particles to repel each other can overcome the adhesion forces holding the toner in place. The toner tends to blow off the copy sheet or to move on the surface of the copy sheet.
One prior art solution to this problem is post-nip ionization as discussed in U.S. Pat. No. 3,781,105, issued to Thomas Meagher. The Meagher patent teaches a no-charge-exchange transfer roller with an electric field that increases from a pre-nip region to a post-nip region. Transfer is accomplished in the area of the nip between the transfer roller and the photoconductor. The electric field in the post-nip region is high enough to ionize the air between the transfer roller and the back (side opposite from toner) of the transfer sheet. Thus, the Meagher apparatus deposits charge on the back of the paper or transfer sheet as the sheet leaves the photoconductor. This charge on the transfer sheet holds the toner on the transfer sheet after the transfer. The difficulty with the Meagher apparatus is that it is very difficult to manufacture transfer rollers that will have the desired electric field characteristic that Meagher calls for. Further, the Meagher apparatus is very sensitive to atmospheric conditions such as pressure and humidity.
To review, Meagher charges the back side of the paper as it is leaving the transfer roller so as to help the paper retain the toner particles. The difficulty with this approach is that atmospheric conditions may affect the quality of the charge on the paper and may also cause charges to migrate to the photoconductor thus defeating the no-charge-exchange transfer process.